Because many congenital heart defects cannot be corrected entirely at birth, palliative heart surgery is performed in order to extend the life of an infant until such time that corrective heart surgery can be performed. "Cyanotic defects" is one of the categories of congenital heart defects for which palliative surgery is normally performed. A cyanotic defect is one in which malformations within the heart reduce pulmonary blood flow resulting in low oxygen saturation levels within the blood. This defect often causes a bluish hue in the patients due to the low arterial oxygen saturation levels. In many cases, children born with cyanotic congenital heart disease cannot have the defect repaired at birth, but must wait for at least one year before corrective surgery, such as a Fontan procedure can be performed.
Palliative surgery, however, is usually performed immediately on the infant in order to at least partially alleviate the low oxygen saturation level. For example, for certain cyanotic congenital heart diseases, such as tricuspid atresia, a systemic-pulmonary artery shunt is installed in order to provide a means for conveying blood from the aorta to the pulmonary system where the blood is oxygenated by the patient's lungs. Palliative surgery for non-cyanotic heart defects, such as a "Norwood" procedure for hypoplastic left heart syndrome, may use a systemic-pulmonary shunt as part of the repair.
Most shunts currently used in treating cyanotic heart disease consist of simply prosthetic tubes made from Gore-Tex.RTM.. It is believed that the pulmonary blood flow through a shunt must be maintained within certain limits to avoid adverse side effects. Excessive pulmonary blood flow may increase pulmonary vascular resistance, may burden the ventricle with volume overload, and may ultimately decrease the chance for a successful Fontan procedure later. Insufficient pulmonary blood flow will result in low oxygen saturation levels in the blood.
Attempts have been made to indirectly control the flow rate of blood through a systemic-pulmonary shunt. These attempts have traditionally utilized systemic vasoconstrictors or vasodilitators. Systemic vasoconstrictor will force more blood through the shunt; systemic vasodilitators will have the opposite effect. One recent attempt included use of CO.sub.2 in the inspired gas of the patient in the immediate post-operative period. Decrease partial pressure of CO.sub.2 in the arterial blood results in decreased pulmonary vascular resistance. Varying the concentration of CO.sub.2 in the inspired gas allows one to vary the pulmonary vascular resistance and therefore, exert some control over the pulmonary systemic blood flow ratio.
The above methods imprecisely control the systemic pulmonary blood flow ratio and have other hemodynamic side effects. These methods are also only applicable in the intensive care unit setting. While they may help in the early post-operative period, they do not offer any control of shunt flow from the time of leaving the intensive care unit until the time of a Fontan or hemi-Fontan procedure. It is hypothesized that precise control of blood flow through the shunt from the time of palliation until the Fontan or hemi-Fontan procedure can be performed, is desirable to ameliorate the side effects of increased pulmonary vascular resistance and volume overload of the ventricle.